Chromatin plays two essential, yet by their very nature mutually exclusive roles. On one hand, chromatin is responsible for the extreme degree of DNA compaction that allows the eukaryotic genome to fit into the confines of the nucleus. On the other hand, it must permit the local unraveling of DNA to grant access of the cellular machinery to the genome. Activities that mediate the inter- conversion between various levels of compacted chromatin states are key regulators of all processes requiring access to genomic DNA. Mechanistic and structural insight into these biologically relevant processes is limited. Here, the role of the nucleosome assembly protein 1 (NAP1) family of histone chaperones in maintaining and modulating chromatin structure and fluidity will be investigated. Aims 1 and 2 address the structural and molecular basis for chaperone interaction with histones, using x-ray diffraction as well as biochemical and in vivo approaches. The mechanism and biological role of NAP1-mediated histone removal and histone exchange will be studied by rigorous kinetic analysis combined with in vivo examination of chromatin structure (aim 3). Finally, the novel hypothesis that a posttranslational modification regulates NAP1 activity in metazoans will be put to test in aim 4. The strength of this proposal lies in a combination of a broad spectrum of structural, biophysical, biochemical and genetic approaches to address the highly significant question of how compacted DNA is made available to the cellular machinery.